Theresa May has slapped down demands by the EU that the Brexit negotiations are conducted in French, as she rebuked European leaders for their lack of maturity.

The Prime Minister used her first European Council meeting to dismiss calls by Michel Barnier, the EU's lead Brexit negotiator, for the "working language" of the exit talks to be in French.

She also suggested that "immature" EU leaders are looking for "problems" rather than "opportunities" during the Brexit talks. And she made clear that she intends to negotiate trade deals ahead of Britain's formal exit from the EU, despite threats of legal action from Jean-Claude Juncker, the president of the European Commission.

Mr Barnier was on Friday reported to have demanded that EU and British officials conduct all major Brexit talks in French, in what was seen as an attempt to humiliate Mrs May at her first Brussels summit.

Asked about his view, Mrs May said: "We will conduct the negotiations in the way that means we are going to get the right deal for Britain."

Aides made clear that Mrs May will refuse any attempt by the EU to use French during the negotiations. Minutes after Mrs May's angry response, Mr Barnier tweeted that he is yet to decide in which language the negotiation will be conducted.

European People's Party leader Manfred Weber expressed anger that the UK was blocking some long-term EU defence measures.

He told the BBC Radio 4 Today programme: "When somebody wants to leave a club, it's not really normal that such a member who wants to leave a club wants to decide about the future of this club. That is really creating a lot of anger, the behaviour of the British Government.

Theresa May Credit:
Roge/Belga via/REX/Shutterstock

On arrival, Mrs May said: "The UK is leaving the EU, but we will continue to play a full role until we leave and we will be a strong and dependable partner after we have left."

"It's in the interests of both the UK and the EU that we continue to work closely together, including at this summit."

Later today she will hold talks with Jean-Claude Juncker, the president of the European Commission, who is expected to criticise her over signals that the Government is pushing for a clean break from Brussels rather than a compromise solution.

A 'nervy performance'

That was a surprisingly nervy performance by Theresa May. She gave short, sharp answers on the Brexit question, but - unsurprisingly perhaps - struck a notably more conciliatory tone in Brussels than she did at the Conservative Party Conference in Birmingham earlier this month.

EU leaders - who were both alarmed and infuriated by Mrs May's tone at that conference - will be encouraged to hear Prime Minister acknowledge publicly that the forthcoming negotiations will required "give and take" on both sides, to create a deal in both sides' interests.

On the key question of whether Britain will have access to the Single Market, or membership of it - two very different things - Mrs May is still fudging.

She said that she wanted Britain to be free to trade "with and within" the single market, which neatly covers both bases.

Osborne: I made mistakes

George Osborne has admitted that he made "mistakes" during the EU referendum and failed to realise that millions of people were furious with the Government.

He told the BBC's Sunday Politics North West that being a backbencher "is a complete change of life and tempo", adding he now has time to "think about mistakes I've made and how we can put them right."

"I've got to go on learning, not least why the country voted the way it did in the referendum that led to me leaving Downing Street."

"I don't think I properly understood the sense that people had in many communities, particularly in the north of England, that they were completely disconnected from the system, from the way our country was governed, they felt angry about things."

EU believes Article 50 declaration could be delayed

There remains a significant EU constituency that believes that Britain will ultimately balk at Brexit when it is confronted with the reality of the hard choices that the EU 27 say they are determined to impose upon the UK.

In the latest iteration of this game, an EU diplomatic source who has been sitting in on the talks reports that Michel Barnier, the European Commission's chief negotiator is letting it be known he thinks Theresa May could delay declaring Article 50 until May next year, at least.

This idea has been fueled, part, by the fact that Mrs May reportedly did not provide a hard date for invoking Article 50 when she spoke at last night's dinner.

The Team Barnier thinking is that now Mrs May fully realises there will be no pre-negotiation - symbolised by the other 27 EU leaders not responding to her speech last night on Brexit - the British government will need more time to put together it's opening gambit. That may, of course, just be a ruse to keep the pressure on the British side not to slip backwards on the timetable.

A UK Government spokesperson is quick to shoot down that idea, however. "There is no change to the timing."

Europe tries to save Canada trade deal

While Britain frets about Brexit, the Europeans are frantically trying to save the EU-Canada trade deal which is being held up in the Walloon parliament in Belgium.

An EU diplomat tells me that Canada has said that if the deal isn't passed today, the Canadian PM will not travel to Europe for a signing ceremony next week. Time is running out, and half the European Commission are now in Walloonia trying to sort it out.

For European's it is a dispiriting display of the EU's governance problems, but also a reminder to the UK that our own EU free trade deal will have to go through a similar ratification process.

Lunch will be an 'exchange of pleasantries'

Senior EU diplomatic sources have told The Telegraph that the lunch between Mrs May and Mr Juncker will on one level be an “exchange of pleasantries” at which the European Commmission president formally welcomes the new British Prime Minister “to the family”.

However the EU is also looking to see if Mrs May will “clarify” her speech to the Tory Party Conference earlier this month which were widely seen in Europe as hostile and pointing towards the likelihood of a so-called ‘hard Brexit’. “I am sure for her it will be an opportunity to go through what we have been reading in the press as to what she meant to say - or not - in her speech,” the source added.

UK officials say that Mrs May will not give much away - she is sticking to her mantra of not making any final decisions until Article 50 is declared - but will reiterate that the UK wants the "best possible arrangement between the UK and the EU as close partners" after Brexit.

Lord Hill cautions against 'stupid Brexit'

The UK's former Brussels commissioner has warned against a "stupid Brexit".

Lord Hill told the BBC's Today programme: "We have this kind of false choice in the UK - often between hard Brexit and soft Brexit. I think the choice is between stupid Brexit and more intelligent Brexit, and that's what we need to go for."

He also cautioned that the EU still believes Britain is ready to backtrack on the Brexit vote.

He said: "I think there is a surprisingly widely-held view that Britain might still decide to stay in. And I think that partly explains why previously Donald Tusk has talked about it as hard Brexit or no exit.

"I think they also believe when people start to look at the practical consequences of disentangling ourselves from this very complicated relationship, then maybe we will think again.

"I think it is so important that over the months and years ahead we have as many bridges as possible, and try and work through this in a way that secures what is in our mutual best interest, rather than the danger that I can see is that we end up doing something that is mutually self-harming."

She told her counterparts directly that Britain would be leaving after suggestions in Europe that the referendum result could be revisited.

But at the end of the summit dinner last night, Mr Tusk told reporters: "It's not our choice and if you ask me I would prefer 28 members not only for the next month, but also for the next years and decades.

"After the decision in the UK we have to respect the decision of the referendum. If it is reversible or not, this is in the British hands.

"I would be the happiest one if it reversible but we now we have to start our formal works."

Donald Tusk, president of the European UnionCredit:
Jasper Juinen/Bloomberg

European People's Party leader Manfred Weber expressed anger that the UK was blocking some long-term EU defence measures.

He told the BBC Radio 4 Today programme: "When somebody wants to leave a club, it's not really normal that such a member who wants to leave a club wants to decide about the future of this club. That is really creating a lot of anger, the behaviour of the British Government.

"It's about the long-term project of the European Union, and the Brits decided not to stay in.

"I think it's totally understandable if we, as Germans, as French, as Italians, think about our - not your - long-term project, please don't stop it, don't block it, because that will have a lot of impact on the Brexit negotiations if you do so.

"It's a question of behaviour, it's a question of respect. You want to go away from the centre of the European debate.

“It is not about institutions in Brussels, its about whether the British prime minister in the next years is still at the table, and the majority decided to not sit anymore on the centre of the European debate, and that is not good.”